Pulmonary embolism is a recognized medical emergency, and may be caused by venous thrombosis. The venous thrombosis may be caused by blood flow retention, venous intima damage, or coagulation abnormalities. Recognized treatments include administration of anti-coagulant medication therapy, thrombolytic therapy, thrombectomy, and inferior vena cava thrombosis filtering procedures. When an inferior vena cava thrombosis filtering procedure is selected, it can be accomplished using either a laparotomy procedure under general anesthesia, or percutaneously inserting a thrombosis filter under local anesthetic. A laparotomy procedure is a surgical procedure done under general anesthesia, and is susceptible to thrombosis formation due to discontinuance of anti-coagulant therapy prior to such surgery.
A recognized option is to intravenously insert a thrombosis filter in the vascular system, and in particular into the inferior vena cava, which requires only a local anesthetic. Percutaneous filter insertion has been recognized as an effecateous procedure since only a local anesthetic is required; however, such thrombosis filters have been recognized to become affixed to the inner vena cava wall or vein wall by neointimal hyperplasia within a relatively short time after implantation. This process can occur within two or three weeks, and in prior art filter arrangements renders the filter unremovable by a single percutaneous process without incurring significant vessel trauma.
There are a number of thrombosis filters which have been developed with the intent of allowing percutaneous removal. Those prior art thrombosis filters that include substantially linear struts tend to distribute forces along the longitudinal axis of the struts. With the struts deployed outwardly to engage the walls of the lumen, asymmetrical compression of the lumen can cause the struts to be forced together in a manner that causes the struts to do damage to the lumen wall.
As indicated, there are a number of prior art implantable filters. One example is the filter disclosed in U.S. Pat. No. 4,817,600 issued to James Kay Herms, et al., which describes a set of shaped leg portions that are joined at one end and are arranged at an acute angle to the axis of the filter, and form a generally conical arrangement. The shaped legs include hooks at the extremity for hooking into the vein wall and holding the filter in position against the flow of blood within the lumen. Herms, et al. provided for an improved leg structure that would avoid some of the concerns of the filters that used relatively straight struts, and minimized the damage that could occur to the vessel or lumen arising from tipping or tilting of the filter. It did not, however, describe a structure or method for percutaneously removing the filter.
U.S. Pat. No. 4,990,156 to J. Lefebvre describes a filter that may be percutaneously inserted for temporary use in determining whether or not a more permanent filtering treatment is necessary. The device describes a non-aggressive contact of the filter elements with the vessel and describes a number of elements that each have sharpened and roughened portions contacting the vessel wall and holding the filter in position. A sheath is provided to allow removal should the filter not be required for permanent usage. Once deployed, the filter is positioned for definitive use and may not thereafter be readily removed.
U.S. Pat. No. 5,324,304 issued to Erik Rasmussen, describes another form of implantable filter that is self-expandable and can be inserted through use of a catheter which encloses the structure. The anchoring legs are designed to have hooks at the ends for engaging the wall of the vein once deployed. The anchoring elements form part of the filtering structure, and once placed would tend to hook firmly into the vein walls. No structure or method is described for percutaneous removal.
U.S. Pat. No. 5,370,657 to Toshiyuki Irie describes a recoverable thrombosis filter having a structure wherein the holding mechanism and the filtering mechanism is comprised of a number of opposed elements that are held in place by an intermediate tension member. It recognizes that removal may be desirable, and has described a series of shaped end portions that cooperate with the wall of the vessel, without piercing it deeply. For removal, it is necessary that dual percutaneous procedures be worked from opposite ends of the filter. A pair of hooking devices are engaged from the opposite ends, and the two halves of the filter are stretched apart until the connecting tension member breaks. While the two halves of the filter are drawn within a pair of sheaths for withdrawal, this removal procedure requires that two opposed removal structures be administered to the opposite ends of the filter, and that manipulation of the two removal devices be coordinated to grasp the opposed hooking elements such that the filter can be broken in half and withdrawn. This removal process doubles the risk to the patient, and due to the small size of the elements, is relatively difficult to accomplish.
The foregoing described prior art is illustrative of various types of filter structures and handling devices that are known for use in placing and removing thrombosis filters. The prior art structures do not describe filter structures that are readily removable utilizing a single percutaneous removal procedure.
To address the deficiencies in the prior art, the present invention was developed to provide an improved recoverable thrombosis filter that can be removed through a percutaneous procedure even after having been in place for such time as to have had neointimal hyperplasia to have fully developed. Through the use of a unique holding structure and a filtering portion of the thrombosis filter, the filter is structured such that the holding portion can be collapsed from one end through external manipulation and the entire filter drawn within an enclosing structure for removal. These and other more detailed specific objectives of the invention will become apparent to those skilled in the art from consideration of the drawings and the description of the preferred embodiments.